Field of the Invention
The invention relates to a projection exposure method for exposing a radiation-sensitive substrate with at least one image of a pattern of a mask, and to a projection exposure apparatus suitable for carrying out the method.
Description of the Prior Art
Microlithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely patterned components. These methods involve the use of masks (photomasks, reticles) that bear or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The mask is positioned in a projection exposure apparatus in the beam path between an illumination system and a projection lens such that the pattern lies in the region of the object plane of the projection lens. A substrate to be exposed, for example a semiconductor wafer coated with a radiation-sensitive layer (resist, photoresist), is held in such a way that a radiation-sensitive surface of the substrate is arranged in the region of an image plane of the projection lens, the image plane being optically conjugate with respect to the object plane. During an exposure process, the pattern is illuminated with the aid of the illumination system, which, from the radiation of a primary radiation source, shapes an illumination radiation which is directed onto the pattern and which is characterized by specific illumination parameters and impinges on the pattern within an illumination field of defined form and size. The radiation altered by the pattern passes as projection radiation through the projection lens, which images the pattern onto the substrate which is to be exposed and is coated with a radiation-sensitive layer. Microlithographic projection exposure methods can e.g. also be used for producing masks (recticles).
One of the aims in the development of projection exposure apparatuses is to produce structures having increasingly smaller dimensions on the substrate via lithography. Smaller structures lead to higher integration densities e.g. in the case of semiconductor components, which generally has a favorable effect on the performance of the microstructured components produced. The size of the structures that can be produced is crucially dependent on the resolving power of the projection lens used and can be increased firstly by reducing the wavelength of the projection radiation used for projection, and secondly by increasing the image-side numeral aperture NA of the projection lens that is used in the process.
In the past, refractive projection lenses have predominantly been used for optical lithography. In the case of a refractive or dioptric projection lens, all of the optical elements which have a refractive power are transparent refractive elements (lens elements).
In order to ensure a sufficient correction of aberrations, in particular chromatic aberrations, and of the image field curvature, even at shorter wavelengths, catadioptric projection lenses are increasingly being used, that is to say projection lenses containing both transparent refractive optical elements having a refractive power, that is to say lens elements, and reflective elements having a refractive power, that is to say curved mirrors. Typically, at least one concave mirror is contained.
Furthermore, optical systems for microlithography have been developed which operate with moderate numeral apertures and obtain the increase in the resolving power essentially by virtue of the short wavelength of the used electromagnetic radiation from the extreme ultraviolet range (EUV), in particular having operating wavelengths in the range of between 5 nm and 30 nm. Radiation from the extreme ultraviolet range (EUV radiation) cannot be focused or guided with the aid of refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at higher wavelengths. Mirror systems are therefore used for EUV lithography. The masks used are reflective masks.
Projection lenses generally have a multiplicity of optical elements in order to make possible in part contrary requirements with regard to the correction of imaging aberrations if appropriate also in the case of large numeral apertures used. Both refractive and catadioptric imaging systems in the field of microlithography often have ten or more transparent optical elements. In systems for EUV lithography, it is endeavored to manage with the fewest possible reflective elements, e.g. with four or six mirrors.
The optical elements are held with the aid of holding devices at defined positions along a projection beam path of the projection lens. Lens elements and other optical elements are often held via a multiplicity of holding elements arranged at the circumference of the respective optical element. In this case, the optical element has an optical used region lying in the projection beam path and an edge region lying outside the optical used region, wherein one or a plurality of holding elements of the holding device assigned to the optical element engage on the edge region. Refractive or specularly reflective surfaces with optical quality are prepared in the optical used region, while the optical quality does not have to be achieved in the edge region. The optical used region is often also designated as “free optical diameter” of the optical element.
Besides the intrinsic imaging aberrations which a projection lens can have on account of its optical design and production, imaging aberrations can also occur during the period of the use, e.g. during the operation of a projection exposure apparatus by the user. Such imaging aberrations are often caused by alterations of the optical elements incorporated in the projection lens as a result of the projection radiation used in the course of the use. By way of example, a certain part of the projection radiation can be absorbed by the optical elements in the projection lens. The extent of the absorption is dependent, inter alia, on the material used for the optical elements, for example the lens element material, the mirror material and/or the properties of antireflection coatings or reflection coatings possibly provided. The absorption of the projection radiation can lead to heating of the optical elements, as a result of which, in the optical elements, a surface deformation and, in the case of refractive elements, a change in refractive index can be brought about directly and indirectly via thermally induced mechanical stresses. Changes in refractive index and surface deformations lead, in turn, to alterations of the imaging properties of the individual optical elements and hence also of the projection lens overall. This problem area is often dealt with under the key words “lens heating”.
Other internal or external disturbances can also lead to the impediment of the imaging performance. They include, inter alia, a possible scale error of the mask, alterations of the air pressure in the surroundings, differences in the strength of the gravitational field between the location of the original lens adjustment and the location of use by the customer, changes in refractive index and/or form alterations of optical elements on account of material alterations as a result of high-energy radiation (e.g. compaction), deformations on account of relaxation processes in the holding devices, drifting of optical elements and the like.
Attempts are usually made to at least partly compensate for imaging aberrations that occur during the service life, in particular the imaging aberrations that occur during operation, by using manipulators. The term “manipulator” in this case denotes, inter alia, optomechanical devices that are designed to act, on account of corresponding control signals of an operating control system, actively on individual optical elements or groups of optical elements in order to alter the optical effect thereof, in particular to alter it in such a way that an aberration that occurs is at least partly compensated for. The term “manipulator” also encompasses devices which, on account of corresponding control signals of an operating control system, act on the mask or on the substrate in order, for example, to displace, to tilt and/or to deform the mask or the substrate. A manipulator can be designed e.g. for decentering an optical element along or perpendicular to a reference axis, tilting an optical element, locally or globally heating or cooling an optical element, and/or for deforming an optical element.
A manipulator contains one or a plurality of actuating elements or actuators, the present actuating value of which can be changed or adjusted on account of control signals of the operating control system. If an actuating value change involves a movement of an actuator, e.g. in order to displace or to tilt an optical element, then an actuating value change can also be designated as “manipulator travel”. An actuating value change can also be present e.g. as a temperature change or as a change in an electrical voltage.
High-productivity projection exposure apparatuses for microlithography comprise an operating control system which makes it possible to perform a near-instantaneous fine optimization of imaging-relevant properties of the projection exposure apparatus in reaction to environmental influences and other disturbances. For this purpose, appropriately to the present system state, at least one manipulator is driven in order to counteract a disadvantageous effect of a disturbance on the imaging performance. In this case, the system state can be estimated e.g. on account of measurements, from a simulation and/or on the basis of calibration results, or can be determined in some other way. In this case, in general information concerning the present use is also taken into consideration, which includes in particular, information about the diffracting and/or the phase altering structure of the pattern to be imaged and/or information concerning the illumination mode used (illumination setting).
The actuating value changes on manipulators, or on actuators of manipulators, required for a desired intervention in the system are determined in known operating control systems on the basis of a control program with a correction algorithm that optimizes a target function (merit function). What is thus intended to be achieved, inter alia, is that, rather than an individual residual aberration being minimized at the cost of others, an expedient, balanced reduction of all relevant influencing variables to values that can be afforded tolerance is achieved.
The European Patent EP 1 251 402 B1 describes an operating control system that uses a target function. In this case, the target function describes the quality of the exposure process as a weighted sum of a multiplicity of “lithographic aberrations”. In this case, the term “lithographic aberration” is intended to encompass all defects relevant to lithography during the imaging. The lithographic aberrations include, inter alia, aberrations such as distortion, deviations of the lateral image position, image rotation, asymmetrical magnification, deformations of the focus position, etc., but also variations of the critical dimensions over the image field (CD variations), differences in the critical dimensions in mutually orthogonal directions (HV aberrations), etc. These lithographic aberrations are influenced by various properties of the projection exposure apparatus or of the projection exposure process, including the substrate, the radiation-sensitive layer on the substrate, the projection ray provided by the light source, the mask and the projection system.